Polyamide substrates, such as nylon carpeting, upholstery fabric and the like, are subject to staining by a variety of agents, e.g., foods and beverages. An especially troublesome staining agent is FD&C Red Dye No. 40, commonly found in soft drink preparations. Different types of treatments have been proposed to deal with staining problems. One approach is to apply a barrier such as a highly fluorinated polymer to the substrate. Another is to use stain resists consisting of either phenol-formaldehyde condensates or acrylic acid as well as styrene/maleic acid polymerisates or combination of these. Examples of phenol-formaldehyde condensates are described in U.S. Pat. Nos. 4,501,591, 4,592,940, 4,680,212, 4,833,009 and 4,963,406. These materials are either condensation products of naphthalene monosulfonic acids with BHPS and formaldehyde or condensation products of phenolsulfonic acid with BHPS and formaldehyde. To impart stain resistance and solubility, the presence of sulfonic acid groups is necessary in these compounds. Stain-resists of this type have to be applied below pH of 4.5 , preferably below 3.0. Examples of poly(acrylic acid) type stain-resists are described in U.S. Pat. Nos. 4,940,757 , 5,015,259 and EUR 332,342, and of styrene/maleic acid type stain-resists in U.S. Pat. Nos. 4,883,839, 4,948,650, and 5,001,004. Stain-resists of this type have to be applied below pH of 3.0, preferably below 2.5.
Applications of stain-resists at low pH's cause corrosion of equipment and safety as well as environmental problems. Therefore, the ability of a stain-resist to function in a neutral or close to neutral environment is highly desirable. The use of mercapto acids instead of sulfonic acids in the condensation of BHPS and formaldehyde is quite different from the prior art. The synthesis of monomeric carboxyalkylthiomethylated phenols has been described in Synthesis(April 1989, page 253). These materials have no stain-resist properties. The synthesis of the stain-resists described in this invention when carried out above 100.degree. C. and at specific molar ratios leads to unique and unexpected properties.